Numerous genes conferring resistance to pathogens have been identified and used in plant breeding. However, single-gene pathogen resistance in plants often becomes ineffective due to the emergence of new virulent races of the disease agent. In contrast, durable disease resistance in plants is generally thought to be controlled by multiple genes. A few rust resistance genes have been isolated and cloned from wheat (Feuillet et al., 2003; Huang et al., 2003; Cloutier et al., 2007) and other cereals (Collins et al., 1999; Brueggeman et al., 2002) and are predominantly from the nucleotide binding site-leucine rich repeat (NB-LRR) class of major resistance (R) genes. For example, three wheat R genes (Lr1, Lr10 and Lr21) that provide protection against the wheat leaf rust fungus, Puccinia triticina, have been cloned (Somers et al., 2004; Hayden et al., 2008; Manly et al., 2001). One exception is the barley Rpg1 rust resistance gene which encodes a protein kinase. These genes encode gene-for-gene resistance against single pathogens and generally lead to strong, hypersensitive responses in the plant tissues upon infection.
In contrast, rust resistance genes in wheat (Triticum aestivum L.) such as Lr34, located on the chromosome arm 7DS, confer a broad spectrum and durable adult plant resistance against several obligate biotrophic pathogens including fungi from the Ascomycetes and Basidiomycetes. These include leaf rust, stripe rust, stem rust and powdery mildew and therefore the Lr34 gene has been widely deployed in wheat breeding despite its weaker, non-hypersensitive response phenotype (Dyck, 1977 and 1987; German and Kolmer, 1992; Bossolini et al., 2006; Spielmeyer et al., 2008). Cultivars with the resistance locus Lr34 such as Frontana have had effective durable resistance to the leaf rust fungus Puccinia triticina Eriks (Dyck et al., 1966; Singh and Rajaram, 1994). To date, isolates of P. triticina with complete virulence to Lr34 have not been detected (Kolmer et al., 2003). The Lr34 gene was recently cloned and shown to encode a protein in the ABC transporter family (Krattinger et al., 2009), although its function as a transporter was not demonstrated. Lr34 resistance has remained genetically inseparable from the gene designated Yr18 that confers resistance to stripe rust (P. striiformis) (Singh, 1992; McIntosh, 1992). Co-segregation of Lr34/Yr18 with other traits such as leaf tip necrosis (Ltn1) in adult plant stage, powdery mildew (recently designated Pm38), tolerance to barley yellow dwarf virus (Bdv1) and spot blotch (Bipolaris sorokiniana) have been documented (Singh, 1992a,b; McIntosh, 1992; Joshi et al., 2004; Spielmeyer et al., 2005; Liang et al., 2006), and these phenotypes are all thought to be conferred by the Lr34 resistance polypeptide.
A second gene that confers broad spectrum, adult plant resistance against several obligate biotrophic pathogens is Lr67, located on chromosome 4DL in wheat, and found in a few wheat accessions such as RL6077 (Herrera-Foessel et al., 2011). In contrast to Lr34, the Lr67 gene has not been widely used to produce resistant cultivars for commercial wheat production. Although an initial report (Dyck et al., (1994) based on plant phenotypes suggested that the resistance gene in RL6077 might be a translocated Lr34, this was subsequently shown not to be the case (Herrera-Foessel et al., 2011). After mapping the gene in two segregating populations, Hiebert et al., (2010) designated the gene in RL6077 as Lr67. Although Lr67 also leads to leaf tip necrosis and provides a partial, broad spectrum, adult plant resistance to leaf rust and stripe rust like Lr34, these are clearly different genes.
There is a need to determine the molecular basis of genes such as Lr67 that provide quantitative non-race-specific, adult plant pathogen resistance-type or partial resistance to a broad spectrum of pathogens.